Purification of argon through liquid phase cryogenic adsorption

ABSTRACT

The invention relates to a process for removing oxygen from liquid argon using a TSA (temperature swing adsorption) cyclical process that includes cooling an adsorbent bed to sustain argon in a liquid phase; supplying the adsorbent bed with a liquid argon feed that is contaminated with oxygen and purifying the liquid argon thereby producing an argon product with less oxygen contaminant than is in the initial liquid argon feed; draining the purified residual liquid argon product and sending purified argon out of the adsorbent bed. Regeneration of specially prepared adsorbent allows the adsorbent bed to warm up to temperatures that preclude the use of requiring either vacuum or evacuation of adsorbent from the bed.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is a continuation application of U.S. Patentapplication Ser. No. 13/782,636 filed on Mar. 1, 2013, now issued U.S.Pat. No. 9,222,727, and entitled PURIFICATION OF ARGON THROUGH LIQUIDPHASE CRYOGENIC ADSORPTION.

FIELD OF THE INVENTION

The present invention relates to the use of cyclic adsorption processesfor the removal of oxygen required for the purification of liquid argon.More specifically, the invention relates to the process steps,conditions, and adsorbents to purify a liquid argon stream of oxygen.The present invention also describes an optimal and economicallyattractive lower energy consumption process for obtaining a commerciallyviable liquid argon product. In addition, the invention also providesthe identification of an optimal adsorbent for use in this purificationprocess. This purification process can be integrated with an airseparation plant or unit (ASU), under field service relevant conditions.

DESCRIPTION OF RELATED ART

Successful development of a cyclic adsorption process to achieve removalof low concentrations (i.e., in the range of parts per million) ofoxygen from liquid argon, requires the identification of a suitableadsorbent as well as the development and optimization of the adsorptionprocess steps.

The removal of low concentrations of oxygen from argon is considered tobe a purification process and is necessary for many end users of argonwhere the presence of oxygen in the argon is undesirable. In manyinstances where safety, handling, and the industrial or laboratory useof argon in either a liquid or gaseous state occurs, the purity of argonis important. Argon is colorless, odorless, and nontoxic as a solid,liquid, and gas. Argon is chemically inert under most conditions. As aninert noble gas, it possesses special properties desirable forapplications related to the semi-conductor industry, lighting, and othertypes of gas discharge tubes, welding and other high-temperatureindustrial processes where ordinarily non-reactive substances becomereactive. Oxygen, in contrast to argon, is a highly reactive substance(in gaseous or liquid form) and is often a safety concern in that itsupports combustion. Even low levels of oxygen (<100 parts per million)are many times not acceptable for certain laboratory and industrialprocesses. This also includes the chemical processing industry wherecertain reactions must be carried out primarily in the absence ofoxygen. Cost considerations for the purification of argon have been adriving influence in the development of special cryogenic systems overat least several decades, and finding a suitable process which isrobust, reliable, and meets the economic criteria necessary for customerdemand has been sought. Production of liquid argon via cryogenicdistillation is well known and is the preferred method of producing highpurity argon.

Adsorption processes have also been described for the purification ofargon, however, these have in general been limited to gas phase using 4Aadsorbents and involved expensive energy intensive adsorption processes.For example, considerable cost is added to the adsorption processwhenever an evacuation step is required. The adsorption process step ofregeneration that requires vacuum has been historically very energyintensive in that vacuum processing requires special equipment and otheradditional peripherals leading to much higher energy demands as well asthe addition of undesirable but necessary capital and operatingexpenses.

In the related art, U.S. Pat. No. 3,996,028 provides for purification ofargon using an adsorption process to remove oxygen impurities by passinga contaminated argon stream through synthetic zeolites of the A type atcryogenic temperatures. The document provides for vacuum treatment as anecessary step for desorption of oxygen from the zeolite following awarm-up regeneration step. Moreover, during the adsorption step theargon feed is in the gaseous phase and, the purified argon productprovided is in the gas phase.

U.S. Pat. No. 4,717,406 describes the on-site adsorption of impuritiescontained in liquefied gases by passing liquefied gases through anactivated adsorbent material at cryogenic temperatures and pressures fora time sufficient to permit adsorption. However, a necessary componentof this process includes filters upstream and downstream of theadsorbent bed. The examples that have been provided in this documentpertain to the purification of liquified oxygen gas from carbon dioxideas this comes in contact with an adsorbent bed which is initially atambient temperature.

U.S. Pat. No. 5,685,172 describes a process for the purification ofoxygen and carbon dioxide from a cold gas or liquid stream of at least90 mol % of nitrogen, helium, neon, argon, krypton, xenon, or a mixtureof these gases. To achieve this, the use of a porous metal oxide, suchas hopcalite-like materials are required. The regeneration of thesemetal oxides requires a reducing agent, such as hydrogen, whichincreases the total operating cost of adsorption processes using thesematerials. The zeolites described in the present invention are differentthan hopcalite and do not require use of reducing agents forregeneration. More specifically, hopcalites are chemisorbents orcatalysts where zeolites, however, are reversible physical adsorbents.In addition, hopcalite materials are largely non-crystalline. Anycrystallinity associated with hopcalite is attributed to the MnO₂component which is present mainly in amorphous form. In contrast,zeolites are crystalline materials.

U.S. Pat. No. 6,083,301 describes a PSA or TSA process for purifyinginert fluids to at most 1 part per billion impurities for use in thefield of electronics. This patent describes the use of hopcalite-likeadsorbent for the capture of oxygen impurities from liquid streams.

U.S. Pat. No. 5,784,898 also describes a cryogenic liquid purificationprocess by which the liquid to be purified is brought in contact with anadsorbent to permit the adsorption of at least one of its contaminants.It is disclosed that at least a portion of the adsorbent is maintainedcold using purified cryogenic liquid in between two subsequentpurification cycles. Clearly, regeneration of the adsorbent is notdescribed as a step that is provided in between the purification cycles.According to U.S. Pat. No. 5,784,898, following the completion of thepurification cycle, the adsorbent is kept cold by coming into directcontact with a portion of the purified cryogenic liquid until the nextpurification cycle. Regeneration of the adsorbent takes place after anumber of purification cycles and after draining the cryogenic liquidfrom the reactor.

In short, there are several limitations associated with the commercialpurification of argon using adsorption techniques that have beendiscussed in the related art for certain applications. These knownprocesses have been deficient in meeting all the criteria addressedabove, namely: delivering argon as a liquid with very low oxygenconcentration in an economic, lower energy consuming process. Anotherdisadvantage is the required use of vacuum, which further increasesenergy demand, capital expenditures, and maintenance, and also furtherreduces the robust nature of any of the currently used or known argonpurification processes. Further drawbacks include the fact the adsorbentsystems which use commercially available zeolites of the 4A type requirerelatively large adsorbent beds to accomplish the purification necessaryand these adsorbent beds must be taken “off-line” for frequentregeneration prior to restarting purification. Additional drawbacksassociated with the related art also include the use of hopcalite-likeadsorbents that do not possess the required physico-chemical propertiesneeded for simple adsorbent regeneration and require the use of hydrogenas a reducing agent which is costly. These related art processes are notoptimal for large scale operation in ASUs that produce up to a couple ofhundred tons of liquid argon on a daily basis in that the TSA process ofthe present invention is a liquid compatible, continuous cyclic process,using a modified zeolite adsorbent.

Unmet needs remain regarding manufacture of large scale liquid argonpurification with low parts per million levels (down to or below 1 partper million is desirable) of oxygen using adsorption technology. Thisincludes the development of an optimal, economic, and effectiveadsorbent regeneration scheme as well as adsorbents with maximumcapacity for oxygen uptake and negligible uptake for argon, whichenables the use of smaller adsorbent beds.

To overcome the disadvantages of the related art, it is an object of thepresent invention to describe a novel process for liquid argonpurification. This includes the use of a Temperature Swing Adsorption(TSA) process. The adsorbent is effectively regenerated by removing mostof the adsorbed oxygen, by purging with a warm nitrogen and/or argonstream to above cryogenic temperatures.

It is also an object of the present invention to provide for a specificcombination of a TSA process cycle along with the use of special formsof zeolite 4A material for providing the most efficient requiredseparation. Some of the related art discloses the use of hopcalitematerials to purify oxygen contaminants from liquid argon (see, e.g.,U.S. Pat. Nos. 5,685,172 and 6,083,301). The use of 4A zeolite materialsis also described in the cited art (e.g., U.S. Pat. No. 3,996,028), butin applications where the purification process takes place in the gasphase and requires a vacuum pretreatment step for the regeneration ofthe adsorbent. In the present invention, there is no need for a vacuumpretreatment step. The purification takes place in the liquid phase, andthe adsorbent has been modified to accommodate the requirements of thenew and unique process.

Other objects and aspects of the present disclosure will become apparentto one of ordinary skill in the art upon review of the specification,drawings, and claims appended hereto.

SUMMARY OF THE INVENTION

The present invention describes a TSA process for removing oxygen fromliquid argon, comprising the following cyclical steps:

a) supplying the adsorbent bed with the liquid argon feed that containsoxygen, thereby producing a purified liquid argon product leaving theadsorbent bed with less oxygen than existing in the liquid argon feed;

b) draining the purified residual liquid argon product and removing thisresidual out of the bed and;

c) allowing the adsorbent bed holding the adsorbent to warm to atemperature such that the absorbent is regenerated to the point that theadsorbent bed can continue to remove the oxygen and continue to providethe purified liquid argon once the adsorbent bed is cooled down asdescribed in step (d) below.

d) cooling an adsorbent bed holding adsorbent to a temperature such thatthe adsorbent bed sustains an argon feed in a liquid phase.

The process described above is a cycle operated in a fashion comprisingsteps (a)-(d) where the cycle is repeated, as needed, and the adsorbentbed contains zeolite adsorbents of either the 4A type zeolites or ionexchanged 4A type zeolites or both and where the ion exchange isaccomplished with lithium ions. According to an aspect of the invention,a TSA cyclic process for the purification of liquid argon is provided incombination with the development and use of specific and specialadsorbents. The adsorbents contained within the adsorbent beds areeffectively regenerated to remove oxygen via desorption by warming thebeds with various gases (e.g., nitrogen, argon or gas mixtures includingpurified air) at temperatures that may reach ambient conditions.

Also, the adsorption process for removing oxygen from liquid argon, maybe further described as follows:

a) supplying from the inlet of an adsorbent bed the liquid argon feedthat contains oxygen in the concentration range of about 10 to 10,000parts per million, adsorbing at least part of the oxygen on theadsorbent thereby producing a purified liquid argon product leaving theadsorbent bed from the outlet with less than or equal to 1 parts permillion of oxygen;

b) supplying a nitrogen purge at the outlet of the adsorbent bed anddraining from the inlet of the adsorbent bed purified residual liquidargon and;

c) continuing the nitrogen purge at the outlet of the adsorbent bed andallowing the adsorbent bed containing the adsorbent to warm to atemperature of at least 200 degrees Kelvin, desorbing at least part ofthe adsorbed oxygen and removing this from the inlet of the adsorbentbed and;

d) supplying a gaseous argon purge of at least 200 degrees Kelvin at theoutlet of the adsorbent bed, so that the gaseous effluent at the inletside of the adsorbent bed is predominantly argon;

e) indirectly cooling the adsorbent bed containing adsorbent, where thebed has an inlet and an outlet, as well as a direct and an indirectcooling means to a temperature below about 150 degrees Kelvin and;

f) directly cooling the adsorbent bed with purified liquid argon to atemperature such that the adsorbent bed sustains an argon feed in aliquid phase, such that g) the process steps (a)-(f) are repeated in acyclical manner

The economic advantages provided by the current invention include thereduction of capital cost of more conventional alternative technologiesaimed at purifying liquid argon from oxygen impurities by use ofadsorption processes. This reduction in capital cost is a result of thecombination of an economically attractive adsorption process cycle,especially as it pertains to the regeneration step (e.g., elimination ofany vacuum regeneration step), and the use of a synthetic zeolitematerial that does not require expensive reducing agents (e.g.,hydrogen) to be regenerated.

BRIEF DESCRIPTION OF THE DRAWINGS

The objectives and advantages of the invention will be better understoodfrom the following detailed description of the preferred embodimentsthereof in connection with the accompanying FIGURE wherein like numbersdenote the same features throughout

The FIGURE illustrates the steps for a cyclic TSA process as provided inthe exemplary embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to and describes a combination of anadsorption process cycle with specific adsorbents to efficiently purifya liquid argon stream into a stream that is primarily free from oxygenimpurities, and methods of making and using the associated process andadsorbent bed.

More specifically, in the present invention, a TSA process has beendeveloped, by which parts per million concentration levels of impuritiesof oxygen are removed from a liquid argon feed stream. The adsorbent forthe TSA process has been selected and prepared so that the on-line timefor each adsorbent bed is on the order of one week prior to anyregeneration requirements. The purified liquid argon product shouldcontain at most 10 parts per million of oxygen, and preferably less thanor equal to 1 part per million of oxygen while the quantity of oxygen inthe liquid feed is usually between 10 and 10,000 parts per million.

The bulk of the oxygen impurity adsorbed in the adsorbent is removed byincreasing the temperature and using a suitable purge gas. The purgeresidual gas (e.g. argon, nitrogen, purified air) loading on theadsorbent, at the regeneration temperature, is substantially low suchthat the adsorbent, after cooling, is still able to remove significantamounts of oxygen from liquid argon streams in subsequent purificationcycles.

The process includes several distinct process steps which are operatedin sequence and repeated in a cyclical manner. Initially the impure(oxygen containing) cryogenic liquid argon is contacted with adsorbentduring the purification or adsorption step, whereupon the oxygenimpurities are substantially adsorbed by the adsorbent and a purifiedliquid argon product is obtained. Next, the oxygen contaminated liquidargon is drained from the adsorbent bed. After the draining of anyresidual cryogenic liquid is complete, the adsorbent bed is warmed to apredetermined temperature that allows for essentially completeregeneration of the adsorbent. Finally, cooling the regeneratedadsorbent within the bed is provided in order that the purificationprocess can begin again. These steps describe a singleadsorption/purification cycle which is repeated as required.

Additionally, several key aspects of the cyclic adsorption/purificationprocess are further described below. First, the process is preferablycontinuous and, therefore, the system requires at least two adsorbentbeds; one of which carries-out the adsorption or purification step whileanother bed is being regenerated in preparation for a further adsorptionor purification step. The choice of the number of beds required to keepthe system operational and efficient is not limited and is dictated bysystem installation and process requirements and/or dictated by customeror application needs. It should be understood that the process describedabove often will include two or more adsorbent beds, wherein the processfor purifying liquid argon in each bed is offset from one another.Specifically, for instance, when one adsorbent bed is being providedfeed gas, a second adsorbent bed can be regenerating, a third adsorbentbed may be idle, and a fourth adsorbent bed may be cooling.

The purification step takes place at or below critical cryogenictemperatures to ensure the liquid state of argon feed persists atpressures in the range of 20-150 psig. However, purification atpressures higher than 150 psig, caused by a hydrostatic head pressuregain or pressurization of the feed using rotating equipment or acombination thereof, is an alternative way of practicing this invention.The oxygen level in the impure cryogenic liquid argon feed can rangefrom as low as 10 parts per million to one or more thousand parts permillion (preferably not more than 10,000 parts per million). The liquidargon feed is introduced at the bottom of the adsorbent bed. Thepurified liquid argon, collected at the top of the bed, is thensubsequently sent to a holding product tank. The purification step iscompleted once the oxygen level in the liquid argon product reaches thedesirable purification level of less than or equal to 10 parts permillion and preferably less than or equal to 1 part per million ofoxygen in argon.

Next, the bed is purged with an inert gas to drain the liquid containedin the adsorbent bed prior to regeneration. The inert purge gas can beeither nitrogen, or argon or a mixture of both, or even purified air.The temperature of the inert gas is at least at the preferred gasboiling point and more preferably near ambient temperature, while itspressure is at least 2 psig and more preferably at least 15 psig. Thedraining step is completed once all the liquid that was contained in theadsorbent bed is drained.

Once the draining step is completed, the regeneration step is initiated.During this step, the temperature of the adsorbent bed increases as itis contacted with the purge gas until the bed temperature reaches atleast 200 degrees Kelvin and more preferably around ambient temperature.The purge gas for the regeneration step is preferably either nitrogen orargon or a mixture of both. In cases where nitrogen and/or argon areless readily available other gases can be used to purge the adsorbentbed and regenerate the adsorbent including mixtures of dry carbondioxide and hydrocarbon free air or a mixture of nitrogen and oxygen.Alternatively, the bed can be initially purged with nitrogen followed byan argon purge. The temperature of the purge gas is at least 120 degreesKelvin and more preferably near ambient temperature, while the pressureis at least 2 psig and more preferably at least 15 psig. The temperatureof the purge gas could be higher than ambient temperature, with theproviso that the porous adsorbent has enough thermal stability towithstand a higher temperature purge. In the most preferred embodiment,the purge gas is introduced from the top portion towards the bottomportion of the bed, in a direction counter current to the liquid feedstream. Purging the bed from the bottom portion to the top portion, inthe same direction as the flow of the liquid to be purified arealternative embodiments which can accomplish similar results, with theproviso that the bed is below the fluidization limit or that theadsorbent and the bed is fully contained.

At the end of the regeneration step, the adsorbent bed reaches atemperature of at least 200 degrees Kelvin, and more preferably aroundambient temperature. To proceed to the next purification cycle, the bedshould be cooled to a temperature below the argon boiling point. One wayto achieve this is via indirect cooling, i.e. by flowing liquid nitrogen(at a pressure ranging from about 18-30 psig) or cold gaseous nitrogenor liquid argon through a jacket surrounding the adsorbent vessel untilthe bed temperature, as measured at the center of the bed, has reachedthe preferred temperature. In one embodiment, this temperature isapproximately 90 degrees Kelvin when the pressure of the liquid feed isabout 60 psig. A most preferred way to achieve this is through acombination of two cooling steps. During the first step, indirectcooling is provided to the adsorbent bed, i.e. by flowing liquidnitrogen through a jacket surrounding the adsorbent vessel until the bedtemperature, as measured at the center of the bed, has reachedapproximately 120 degrees Kelvin. Subsequently, during the secondcooling step, the bed is cooled to approximately 90 degrees Kelvin byflowing liquid argon directly through the bed. This liquid argon streamcould either be obtained from the impure liquid argon feed or from aportion of the purified liquid argon product, depending on the choice ofdesign of the process. The subsequent purification step can be initiatedonce the bed has reached a temperature of 90 degrees Kelvin.

The development of a preferred cyclic cryogenic adsorption processdepends to a high degree on the ability to warm and cool the absorbentbed within a specified and optimal time period. It will be understood bythose skilled in the art that for a two-bed process, the time to drainthe adsorbent bed and the heating (for adsorbent regeneration) andcooling time period also provides a key process variable and time framefor the “on-line time” of each absorbent bed. Furthermore, it isdesirable from a process and economics standpoint to not cycle each bedvery frequently. The preferable online time requirement for each bed isat least one week.

There are alternative process methodologies that could be used topractice the present inventive disclosure, however the most preferredembodiment is discussed below, with reference to the FIGURE.

For purposes of explanation and simplicity, the use of a singleadsorbent bed is described and shown in the FIGURE. However, it will beunderstood by those skilled in the art, that the process described willbe provided for two or more beds for the sake of the continuity of theprocess.

With reference to the exemplary embodiment of the FIGURE, the individualconsecutive steps for a cyclic TSA process employed in the presentinvention are shown. In the initial stage of set-up, the absorbent bed(100) is tightly packed with adsorbent material (200). External coolingwith liquid nitrogen is provided via a cooling jacket (300) thatsurrounds the bed. Stage (A) depicts the initial set-up arrangementprior to the beginning of purification, where the adsorbent bed is atabout 90 degrees Kelvin. Stage (B) illustrates the purification step ofthe adsorption process. During Stage (B), the liquid argon streamcontaining oxygen is fed into the adsorbent bed as represented by thearrow (1). The feed is provided at the bottom of the bed. This feedstream (1) is liquid phase argon that contains oxygen impurities in therange of 10 to 10,000 parts per million of oxygen. The pressure withinthe bed during the introduction of the liquid argon feed is about 60psig and the corresponding temperature for this exemplary embodimentensured that the argon feed remained in the liquid phase at therespective process pressure conditions, namely a temperature of about 90degrees Kelvin. The adsorbent is selected so that under the purificationconditions, the absorbent is selective for oxygen. The liquid argonproduct stream (2) is collected at the top end of the bed. Thepurification step is completed once the level of oxygen in the liquidargon product reaches a concentration of 1 part per million. At thisinstance, the online bed should be prepared for regeneration and thesecond bed is brought online to perform the purification.

Prior to regeneration of the adsorbent, the liquid argon volume in thebed is drained as shown in Stage (C). In order to ensure that the bed isdrained properly and in a timely fashion, a purge step is provided usingan inert gas (normally either argon or nitrogen) denoted as stream (3).The temperature of the inert gas is about 300 degrees Kelvin, while itspressure is preferably about 15 psig. The draining step is completedonce all the liquid that was contained in the adsorbent bed is drained.The liquid drain stream (4), as provided and shown, is rich in liquidargon that remained contaminated with oxygen and collected at the bottomof the bed. The liquid nitrogen was also drained from the cooling jacketand vented to the atmosphere.

After bed (100) is drained, the adsorbent is regenerated using a warmpurge gas while the adsorbent remains within the same bed (100). Asillustrated in Stages (D) and (E), a nitrogen purge through the bed wasinitiated in a countercurrent fashion in relation to the feed (i.e. fromthe top portion to the bottom portion of the bed). The temperature andpressure of the nitrogen purge gas, stream (5) and (7), is about 300degrees Kelvin and 15 psig, respectively. The effluent during the purgeStage (D), indicated as stream (6), was predominantly composed ofundesirable oxygen contaminant, and some argon in the nitrogen purgegas. During this step, oxygen is desorbed from the zeolite adsorbent andsome quantity of argon is desorbed as the temperature within theabsorbent bed rises. As the purging continues, and the bed temperatureapproaches the temperature of the purge gas (shown as nitrogen in stream(7)), the gaseous effluent, stream (8), becomes predominantly nitrogen(Stage (E)). The nitrogen purge is completed when the bed temperaturereaches about 300 degrees Kelvin. At that point, the zeolite becomesloaded with nitrogen. To obtain optimum performance for the liquid argonpurification process of this invention, it was necessary to leave mostof the available sites of the adsorbent free and capable of capturing amajority of oxygen impurities. Hence, subsequent to the nitrogen gaspurge, an argon gas purge, indicated by the stream (9) shown, isimplemented (Stage (F)). The temperature of the gaseous argon for purgeis about 300 degrees Kelvin, while the pressure is around 15 psig. Thisis a very important step in the regeneration of the adsorption scheme.During the last part of the regeneration step, (Stage (F)), a gaseouseffluent of nitrogen and argon exits the bed (100), indicated by stream(10). The argon gas purge is completed when the effluent, stream (10) ispredominantly argon gas. At this instance, the argon gas occupies themacropore space of the adsorbent particles as well as the void spacebetween particles within the adsorbent bed.

Cooling the adsorbent begins in Stage (G). During this stage, indirectheat transfer from a liquid nitrogen medium flowing in a jacket (300)surrounding the bed (100) cooled the adsorbent bed to approximately 120degrees Kelvin. The pressure of the liquid nitrogen in the jacket isregulated so that the liquid nitrogen temperature is above the meltingpoint of argon at the process conditions and below the saturation pointof nitrogen. Once the temperature in the middle of the adsorbent bed isabout 120 degrees Kelvin, the direct cooling step is initiated, as shownin Stage (H). This involves direct contact of the adsorbent material(200) with a purified liquid argon stream denoted stream (11). Stream(11) is introduced at the bottom of the adsorbent bed and it cools thebed to the desired temperature for purification of about 90 degreesKelvin. This also facilitates building a liquid head to fill theadsorbent bed with purified liquid argon. At the end of this step thetemperature at the middle of the bed is about 90 degrees Kelvin and thepressure is around 60 psig. This allows for the next purification cycleto begin again at Stage (A).

Hence, in the context of the current invention, a full TSA purificationcycle involves the following steps:

(i) providing the adsorbent bed with either virgin or regeneratedadsorbent—Stage (A)

(ii) purification of the liquid argon feed providing making purifiedliquid argon product—Stage (B)

(ii) drainage of the liquid argon contained in the bed at the end ofpurification step—Stage (C)

(iii) regeneration of the adsorbent via warm-up—Stages (D), (E), and (F)and;

(iv) cool-down of the adsorbent bed—Stages (G) and (H) so that the cyclecan be repeated.

In describing the adsorbent, it is instructive to understand the needfor the proper adsorbent which will adsorb, at most, very small amountsof argon. The ideal adsorbent does not adsorb any argon and also removesimpurities from the argon which are predominantly oxygen impurities.However, in practice, the adsorbents that have been used still have someargon uptake capacity. Herein are described adsorbents specificallydesigned to minimize argon uptake.

The adsorbents that were developed for the present invention areprimarily beads (with predominantly spherical particle geometry) with anaverage particle size of less than or equal to 2.0 mm and morepreferably less than or equal to 1.0 mm. Additionally, the desiredadsorbents have a porosity that is in a range of between 33 and 40percent as measured by mercury (Hg) porosimetry. A binder is used toformulate the beaded absorbent, such that the binder is present at nogreater than 15 weight percent. This binder is preferably purifiedversions of attapulgite, halloysite, sepiolite or mixtures thereof.

Testing to establish the viability of this purification cycle wasperformed in a pilot plant which included an adsorbent bed with atube-in-tube type cooling system. The inner tube, which had an outsidediameter of one inch, was packed with the adsorbent. The outer jacketwas utilized for passive cooling. The length of the bed was either onefoot or three feet. This bed allowed for receiving cryogenic liquid flowinto an inlet section and the delivery of a cryogenic liquid product atthe outlet. The bed was regenerated on-line as is described above.

Description of the Oxygen Breakthrough Test:

Experiments were performed on the pilot plant scale in order tounderstand several factors associated with the importance of theadsorbent particle size and binder type in affecting the performance ofthe liquid argon purification of oxygen impurities. These arecharacterized as “breakthrough-type” experiments. The generalmethodology of a breakthrough test is well-known to those skilled in theart. For the purpose of the present invention, the breakthrough orworking capacity for oxygen (O₂) was determined using an overall massbalance of oxygen in the feed and in the effluent streams at apredetermined oxygen concentration at the outlet. For the purpose of thepresent invention, this concentration is 1 part per million unlessotherwise specified. The dynamic working capacity (or dynamic capacity)of the oxygen adsorbate was established here to represent the ability ofthe adsorbent to remove oxygen contaminants to a certain level. Thedynamic capacity of oxygen was determined from the oxygen breakthroughtest and was used as an indicator of the ability of the adsorbent toremove oxygen from the feed stream. The conditions of the test werecarefully selected to critically evaluate adsorbents for the desiredadsorption capability under realistic process conditions.

The oxygen dynamic capacity was calculated based on Equation (1):

$\begin{matrix}{{\Delta\; O_{2}} = {\frac{m_{i\; n}}{w_{s}}{\int_{0}^{t_{b}}{( {y_{i\; n} - y_{out}} )\ {\mathbb{d}t}}}}} & (1)\end{matrix}$

Where:

m_(in) is the molar feed flow into the bed

y_(in) and y_(out) are the inlet and outlet mole fractions of oxygenrespectively

w_(s) is the mass of adsorbent;

and;

t_(b) is the breakthrough time corresponding to a predeterminedbreakthrough concentration (in this case—1 part per million oxygenunless otherwise specified).

The dynamic capacity inherently captures kinetic effects resulting frommass transfer resistance. For the purpose of this invention, the primarycomponent in the liquid feed of the breakthrough test was argon. Becausethe concentration of argon in the feed stream was overwhelming incomparison to that of the oxygen concentration, the co-adsorption effectof oxygen upon argon was negligible. Conversely, the co-adsorption ofargon might have had a significant effect upon the adsorption of oxygen.The breakthrough method, as described, was a preferred method forestablishing the dynamic capacity for oxygen because argon co-adsorptionand mass transfer effects were automatically incorporated into theresultant oxygen loading. Therefore, the preferred adsorbent is one thatexhibits high oxygen dynamic capacity (long breakthrough times) in thepresence of such inhibiting factors.

The following example is provided to demonstrate the capability of theTSA process, which demonstrates one embodiment of the present invention,i.e., to remove oxygen to concentrations of less than 1 part per millionfrom a liquid argon stream that contains 10 parts per million of oxygenor more.

EXAMPLE 1 TSA Process Cycle Using Sample A

Preparation and Cool-down of Adsorbent Bed:

Sample A (266.58 g), a 42% lithium exchanged on an equivalent chargebasis zeolite 4A, the development of which is described below (seeExample 3), was loaded on a pilot plant bed. The length of the bed wasthree feet and the internal diameter of the bed was 0.88 inches. The bedwas purged with gaseous nitrogen at 15 psig and 300 degrees Kelvinovernight. The nitrogen flow rate was 5 slpm. The gaseous nitrogen flowwas discontinued and a gaseous argon purge was initiated at 15 psig and300 degrees Kelvin with a gaseous argon purge time of no less than 20min. The argon flow rate was 7.2 slpm.

Subsequent to the argon purge, the flow through the bed was discontinuedand passive cooling of the bed was initiated by flowing liquid nitrogeninto the jacket that surrounds the absorbent bed. The bed was cooled forat least 1 hour, or until the temperature as measured by a thermocouplein the middle of the bed, reached at least 120 degrees Kelvin. At thisinstant, purified liquid argon at 20 slpm was introduced from the bottomportion of the bed towards the top portion of the bed for at least a 45minute period or until the bed temperature, as measured by thethermocouple reached 90 degrees Kelvin.

First Purification/Breakthrough Step:

When the bed temperature reached 90 degrees Kelvin, the liquid argonflow was discontinued and the introduction of a liquid argon stream with99 parts per million of oxygen contaminant was initiated. The flow ratecontinued at 20 slpm.

The introduction of the contaminated liquid argon feed (with 99 partsper million of oxygen) into the adsorbent bed marked the beginning ofthe purification step (Stage (B) in the FIGURE). The flow direction ofthe liquid argon feed stream was from the bottom portion towards the topportion of the bed. After 17.1 hours, the oxygen concentration at theoutlet of the bed reached 1 part per million. The dynamic capacity forthis material for oxygen was calculated to be 1.13 weight percentcorresponding to the breakthrough concentration of oxygen of 1 part permillion. After 17.1 hours, the adsorbent and bed was ready forregeneration.

Drainage Step:

Following the end of the purification/breakthrough step above, theremaining liquid argon was pushed out of the bed by flowing nitrogen at5 slpm (Stage (C) in the FIGURE). At the same time, gaseous nitrogen wasallowed to flow in the jacket around the absorbent bed to initiateevaporation of the liquid nitrogen and transition to the following step,which is the warm regeneration.

Regeneration Step:

Following the completion of the drainage step, the regeneration step wasinitiated (Stage (D) in the FIGURE). The nitrogen purge was continuedovernight and the pressure and temperature of the purge stream was keptat 15 psig and 300 degrees Kelvin respectively. After the gaseousnitrogen flow was discontinued, a gaseous argon purge was initiated at15 psig and 300 degrees Kelvin with a gaseous argon purge time of noless than 20 minutes (Stage (F) in the FIGURE). The argon flow rate was7.2 slpm.

Cool-down Step:

Subsequent to the argon purge, the flow through the bed was discontinuedand passive cooling of the bed was initiated by flowing liquid nitrogeninto the jacket that surrounds the absorbent bed (Stage (G) in theFIGURE). The bed was cooled for at least 1 hour, or until thetemperature as measured by a thermocouple in the middle of the bed,reached 120 degrees Kelvin. At this instant, purified liquid argon at 20slpm was introduced from the bottom portion of the bed towards the topportion of the bed for at least a 45 minute period or until the bedtemperature, as measured by the thermocouple, reached 90 degrees Kelvin.

Second Purification/Breakthrough Step:

The adsorbent bed was now fully prepared for proceeding with thesubsequent purification step. The concentration of oxygen in the liquidargon feed was kept at 100 parts per million. The concentration of theoxygen at the bed outlet was 1 part per million after 17.5 hoursfollowing introduction of the liquid feed into the bed. In this case,the dynamic capacity for oxygen was determined to be 1.21 weight percentat a breakthrough concentration of oxygen of 1 part per million.

In comparing the results from the first and second purification steps,it is clear that the regeneration step of the TSA process accomplishedthe goal of reducing and maintaining the oxygen level of the liquidargon product to below 1 part per million of oxygen over essentially thesame period of time. Hence, the same purification performance wasachieved in two consecutive cycles. This indicates that the ability ofthe adsorbent to remove oxygen from an oxygen contaminated liquid argonfeed is fully restored after the described regeneration scheme iscompleted. After the regeneration step, the adsorbent still exhibitsnearly the same oxygen capacity, thus confirming that the combination ofthe proper adsorbent with the proper process steps provides the desiredresultant product using the process in a reproducible manner.

TABLE 1 Summary of Process Performance Data* Initial Final PurificationConcentration Concentration Step Before and of Oxygen of OxygenPurification O₂ Dynamic Capacity After Adsorbent Impurity Impurity StepTime measured at 1 part per Regeneration (ppm) (ppm) (hr) million (wt %)1st Purification 99 1 17.1 1.13 2^(nd) Purification 100 1 17.5 1.21After Regeneration *The average particle diameter of the adsorbent was1.0 mm and the process used is as described and shown in the FIGURE

As shown by the data summarized in Table 1 above, the present disclosureand accompanying invention combines an advantageous adsorption processcycle with the adsorbent that has proper oxygen capacity and selectivityto efficiently purify a liquid argon stream contaminated with oxygen sothat the oxygen levels are reduced and minimized to levels below 1 partper million. The cyclic TSA process is robust in that the oxygen dynamiccapacity of the adsorbent remains essentially the same after subsequentregeneration of the adsorbent. The cyclic purification process isamenable with use of any adsorbent possessing the characteristicsrequired to achieve the purification of the argon by removing oxygen.

The following example describes a TSA process for the purification ofliquid argon from oxygen that is different than that described inExample 1 in that the regeneration step includes a warm nitrogen purgeonly, as opposed to a nitrogen purge followed by an argon purge (asdescribed in Example 1).

EXAMPLE 2 Alternative TSA Process Cycle Using Sample A

Preparation and Cool-down of Adsorbent Bed:

The preparation and cool-down of the adsorbent bed was identical to thatprovided for Example 1, above. Sample A (92.24 g) was loaded on thepilot plant bed. The length of the bed for this example was one foot andthe internal diameter of the bed was 0.88 inches. The bed was purgedwith gaseous nitrogen and argon as described in Example 1.

Subsequent to the argon purge, the flow through the bed was discontinuedand passive cooling of the bed was initiated as described in Example 1.Following the passive cooling step, purified liquid argon at 40 slpm wasintroduced from the bottom portion of the bed towards the top portion ofthe bed for at least a 45 minute period or until the bed temperature, asmeasured by the thermocouple, reached 90 degrees Kelvin.

First Purification/Breakthrough Step:

When the bed temperature reached 90 degrees Kelvin, the liquid argonflow was discontinued and the introduction of a liquid argon stream with1022 parts per million of oxygen contaminant was initiated. The flowrate continued at 40 slpm. The flow direction of the liquid argon feedstream was as described in Example 1. For Example 2, the capacity of theadsorbent bed at full breakthrough was calculated. This calculation wasperformed using Equation (1), above, where t_(b) is now the time thatcorresponds to the full breakthrough, meaning the time when the oxygenconcentration at the outlet of the bed reaches the inlet feed oxygenconcentration (1022 parts per million, for the present example). Fullbreakthrough was achieved after 20.1 hours. The full bed capacity foroxygen was thus calculated to be 16 weight percent. Following the fullbreakthrough, the adsorbent and bed was ready for regeneration.

Drainage Step:

The adsorbent bed was drained from the remaining liquid argon asdescribed in Example 1.

Regeneration Step:

Following the completion of the drainage step, the regeneration step wasinitiated (Stage (D) in the FIGURE). The nitrogen purge at a flow rateof 5 slpm was continued over a whole weekend and the pressure andtemperature of the purge stream was kept at 15 psig and 300 degreesKelvin respectively.

Cool-down Step:

Subsequent to the nitrogen purge, the flow through the bed wasdiscontinued and passive cooling of the bed was initiated as describedin Example 1. After the passive cooling step was completed, purifiedliquid argon at 40 slpm was introduced from the bottom portion of thebed towards the top portion of the bed for at least a 45 minute periodor until the bed temperature, as measured by the thermocouple, reached90 degrees Kelvin.

Second Purification/Breakthrough Step:

The adsorbent bed was now fully prepared for proceeding with thesubsequent purification/breakthrough step. The concentration of oxygenin the liquid argon feed was kept at 997 parts per million. Theconcentration of the oxygen at the bed outlet 20 hours after theintroduction of the liquid feed into the bed was that of the inlet(approximately 997 parts per million). The full bed capacity for oxygenwas calculated to be 10.2 weight percent under the conditions described.

In comparing the results from the first and second full breakthroughsteps, it is clear that the regeneration step of the TSA process did notaccomplish the goal of restoring the initial adsorbent bed capacity foroxygen. The results reported showed a 36 percent decrease in thecapacity of the adsorbent for oxygen following the regeneration methoddescribed in this example. This indicates that the regeneration schemewhich involves a warm nitrogen purge only (as described in Example 2) isinferior and insufficient compared to the regeneration step whichcombines a warm nitrogen purge followed by a warm argon purge (asdescribed in Example 1). Use of only the warm nitrogen purge does notfully restore the adsorbent bed capacity to remove the oxygen impuritiesin the subsequent purification step.

EXAMPLE 3 Preparation of Sample a (42% Lithium Exchange of Commercial1.0 mm 4A+12% Actigel®)

A commercially produced zeolite 4A sample with 12% Actigel® in beadedform, having an average particle size of 1.0 mm was obtained fromZeochem LLC of Louisville, Ky.

On a dry weight basis, 450 g of the commercially produced sample (562 gwet weight) was stirred in a lithium chloride (LiCl) solution (60.71 gLiCl crystals dissolved in 1500 ml deionized water) for 2 hours at atemperature of 90 degrees Centigrade. This exchange was repeated twomore times. After the first two exchanges, the beads were decanted andwashed by stirring in 2000 ml deionized water for 15 minutes at 90degrees Centigrade. Decant and wash steps were repeated two more times.For the final washing step after the third exchange, the beads wereplaced in a 1.0 inch diameter glass column and using a peristaltic pump,20 Liters deionized water were pumped through the column at rate of 80ml/minute at 80 degrees Centigrade. The beads were removed, air dried,screened to the 16×20 mesh size, then activated using a shallow traycalcination method using a General Signal Company Blue M Electric ovenequipped with a dry air purge. The adsorbents were spread out instainless steel mesh trays to provide a thin layer less than 0.5 inchdeep. A purge of 200 SCFH of dry air was fed to the oven duringcalcination. The temperature was set to 90 degrees Centigrade followedby a 360 minute dwell time. The temperature was then increased to 200degrees Centigrade gradually over the course of a 360 minute period(approximate ramp rate=0.31 degrees Centigrade/minute), and then furtherincreased to 300 degrees Centigrade over a 120 minute period(approximate ramp rate=0.83 degrees Centigrade/minute) and finallyincreased to 593 degrees Centigrade over a 180 minute period(approximate ramp rate=1.63 degrees Centigrade/minute) and held therefor 45 minutes. The 1.0 mm (16×20 mesh) product was characterized by Hgporosimetry to assess porosity characteristics. Chemical analysis of theLi exchange product using standard ICP (Inductively Coupled PlasmaSpectroscopy) methods known by those skilled in the art yielded alithium exchange level of 42% for this sample on a charge equivalentbasis.

The following examples provide additional information with regard toexperimental evidence which eventually led to the present invention. Theadvantage of the adsorbents developed and employed versus thosecommercially available and described in the related art is also furtherdeveloped herewithin.

EXAMPLE 4 Samples B and C (Commercial 2.0 mm and 1.7 mm Zeolite 4A)

Samples B and C were obtained from a commercial manufacturer. Thezeolite is known as Zeochem Z4-04 and manufactured by Zeochem L.L.C ofLouisville, Ky. They were manufactured using greater than 12 weightpercent of a clay, non-Actigel® type binder. The average particlediameter of Samples B and C was 2.0 mm and 1.7 mm respectively.

EXAMPLE 5 Preparation of Sample D (Laboratory 0.6 mm Zeolite 4A from 3APowder+12% Actigel® —Nauta Mixing)

Samples D was a zeolite 4A laboratory sample that contained 12 weightpercent of Actigel®, a purified clay binder. This sample was preparedthrough ion exchange of a zeolite 3A product as described below.

On a dry weight basis, 2100.0 g of zeolite 3A powder (2592.6 g wetweight) was mixed with 286.4 g Actigel 208 (364.9 g wet weight) and 63.0g F4M Methocel in a Hobart mixer for 1 hour and 35 minutes. Theintermediate mixed powder from the Hobart mixer was transferred to aNauta mixer having an internal volume of ˜1 ft³ and agitated therein ata speed of 9 rpm. Mixing with the Nauta device was continued, whilegradually adding de-ionized water to form beads having porosity in therange 30 to 35 percent, as measured after calcination using aMicromeritics Autopore IV Hg porosimeter. At the end of this mixingperiod, beads in the target size 0.6 mm (20×40 mesh) were formed. Theproduct beads were air dried overnight prior to calcination using theshallow tray method at temperatures up to 593 degrees Centigrade. Theshallow tray calcination method described in Example 3 was used. Thecalcined beads were subjected to a screening operation to determine theyield. The particles in the 20×40 mesh size range were harvested forfurther processing, including the steps of hydration, sodium (Na) ionexchange, and activation up to 593 degrees Centigrade under dry airpurge.

Sodium exchange of the samples (to a sodium exchange level of at least99 percent sodium on an charge equivalent basis) was achieved using thefollowing procedure: A column ion exchange process was used where thesamples are packed inside a glass column (dimensions: 3-inch i.d.)contacted with sodium chloride solution (1.0 M) at 90 degrees Centigradeat a de-ionized water flow rate of 15 ml/min. A preheating zone beforethe adsorbent packed column ensured that the solution temperature hadreached the target value prior to contacting the zeolite samples. A5-fold excess of solution was contacted with the samples to yieldproducts with sodium contents of at least 99 percent exchange and above.After the required amount of solution was pumped through the columncontaining the samples, the feed was switched to de-ionized water toremove excess sodium chloride (NaCl) from the samples. A de-ionizedwater volume of 50 L and flow rate of 80 ml/min was used. A silvernitrate (AgNO₃) test, familiar to those skilled in the art, was used toverify that the effluent was essentially chloride free, at the end ofthe washing stage. The wet samples were then dried, rescreened to 0.6 mm(Sample D), and activated under dry air purge (flow rate 200 SCFH) usingthe shallow tray calcination method described above.

EXAMPLE 6 Preparation of Samples E and F (Laboratory 1.0 mm and 0.6 mmZeolite 4A from 4A Powder+12% Actigel® —Nauta Mixing)

Samples E and F were zeolite 4A laboratory samples that also contained12 weight percent of the Actigel®binder, however these were prepareddirectly from a zeolite 4A powder. The samples were prepared using aNauta mixer as described below.

On a dry weight basis, 2100.0 g of zeolite 4A powder (2592.6 g wetweight) was mixed with 286.4 g Actigel 208 (364.9 g wet weight) and 63.0g F4M Methocel in a Hobart mixer for 1 hour and 35 minutes. Theintermediate mixed powder from the Hobart mixer was transferred to aNauta mixer having an internal volume of ˜1 ft³ and agitated therein ata speed of 9 rpm. Mixing using the Nauta device was continued, whilegradually adding de-ionized water to form beads having porosity in therange 30 to 35 percent, as measured after calcination using aMicromeritics Autopore IV Hg porosimeter. At the end of this mixingperiod, beads, including those in the target 16×20 and 20×40 mesh sizerange had formed. The product beads were air dried overnight prior tocalcination using the shallow tray method at temperatures up to 593degrees Centigrade, as described in Example 3. The calcined beads weresubjected to a screening operation to determine the yield. The particlesthat were harvested were 10 mm in size (16×20 mesh) for Sample E, and0.6 mm in size (20×40 mesh) for Sample F. Next, the beads were activatedunder dry air purge (flow rate 200 SCFH) using the shallow traycalcination method as described above in Example 3.

Characterization of Samples of Different Size Using an OxygenBreakthrough Test

Tests were conducted with different sized zeolite 4A samples todetermine oxygen breakthrough under identical process conditions asdescribed above. For the test data provided in Table 2, the systempressure was 60 psig and the temperature during the purification processwas controlled at 90 degrees Kelvin. The feed flow rate was 90 standardliters per minute (slpm) and the bed length was three feet. The feedconcentration into the adsorbent bed was targeted to be either 1000 or100 parts per million of oxygen (contaminant) in the liquid argon streamas specified in Table 2. This target was not achieved in all cases dueto insufficient experimental control.

TABLE 2 Oxygen Breakthrough Performance Data Inlet O₂ PurificationDuration Time Adsorbent Concentration to Obtain Outlet Average in LiquidOutlet O₂ Concentration of O₂ at Less Adsorbent Diameter Argon FeedConcentration than 1 part per million Sample Type (mm) (ppm) (ppm)(minutes) Sample B¹ 2.0 925 722 Not achieved Sample C¹ 1.7 910 403 Notachieved Sample D² 0.6 983 0.17 20 Sample C¹ 1.7 90 31 Not achievedSample E² 1.0 100 .03 43 Sample F² 0.6 100 .02 131  ¹= Commerciallyavailable adsorbent ²= Laboratory prepared adsorbent

Table 2 shows that as the size of the absorbent material was reducedfrom 2.0 mm to 1.7 mm and then to 0.6 mm, the exit concentration ofoxygen was reduced from 722 parts per million to 403 parts per millionand then to 0.17 parts per million respectively, while the initial feedconcentration was approximately 1,000 parts per million oxygen in liquidargon.

When the particle size of zeolite 4A was reduced to 0.6 mm (Sample D),the exit concentration of oxygen was 170 parts per billion and the bedallowed for purification of the liquid argon feed to below 1 part permillion for a full 20 minute duration. Therefore, under the processconditions provided above, unless the zeolite 4A particle size isreduced to 0.6 mm, purifying liquid argon to less than 1 part permillion oxygen, is not possible. These results indicate that the processof oxygen removal from a liquid argon stream is limited by the size ofthe absorbent material.

The same conclusion regarding the need to limit the size of theadsorbent can be reached when the feed concentration was initially setto approximately 100 parts per million of oxygen in liquid argon. Underthese conditions, when the 1.7 mm zeolite 4A (Sample C) was used in theadsorbent bed, the outlet concentration of oxygen in liquid argon wasreduced to 31 parts per million. When the 1.0 mm zeolite 4A (Sample E)was provided in the bed, purification of the liquid feed was achievedfor a 43 minute duration. Finally, when the particle size of the 4Azeolite was reduced even more, to 0.6 mm (Sample F), the purificationwas extended to a 131 minute duration.

EXAMPLE 7 Preparation of Sample G (Laboratory 1.0 mm 4A Sample+12%Actigel®—Tilted Rotating Drum Mixing)

Sample G was another laboratory sample developed from zeolite 4A thatalso contained 12 weight percent of Actigel®. This sample was preparedusing a tilted rotating drum mixer as described below.

On a dry weight basis, 9000.0 g of zeolite 4A powder (11029 g wetweight) was mixed with 1227.3 g Actigel 208 (1575.7 g wet weight) in aSimpson mixer-muller for 1 hour and 20 minutes. The mixed powderedintermediate mixed powder was transferred to a tilted rotating drummixer having internal working volume of ˜75 L and agitated therein at aspeed of 24 rpm. Mixing of the formulation was continued while addingde-ionized water gradually to form beads. A recycling operation wasperformed, involving grinding-up and reforming the beads until the beadsexhibited a porosity, as measured by using a Micromeritics Autopore IVHg porosimeter on the calcined product, in the range of 30 to 35percent. At the end of this mixing time period, beads including those inthe target 1.0 mm size (16×20 mesh) range were formed. The product beadswere air dried overnight prior to calcination using the shallow traymethod at temperatures up to 593 degrees Centigrade, as earlierdescribed in Example 3. The calcined beads were subjected to a screeningoperation to both determine the yield and so that those particles couldbe harvested in the 16×20 mesh size range. Finally, the adsorbentparticles were activated under dry air purge (flow rate 200 SCFH) againusing the shallow tray calcination method as earlier described inExample 3.

EXAMPLE 8 Preparation of Sample H (Commercial 1.0 mm 4A+15-20%Non-Actigel® Type Binders)

Sample H was obtained from a commercial manufacturer. This was thezeolite 4A known as Zeochem Z4-01 and manufactured by Zeochem L.L.C. ofLouisville, Ky. It is manufactured using traditional clay non-Actigel®type binders at a content of 15 to 20 weight percent.

Effect of Binder Type and Content in Oxygen Capacity of Zeolite 4A

Table 3 provides additional oxygen breakthrough data. Here thelaboratory prepared zeolite 4A with 12 weight percent Actigel claybinder of Example 7 (Sample G) is compared to a commercial zeolite 4A ofExample 8 (Sample H). The flowrate of the pilot plant during thepurification step was 20 slpm for these breakthrough tests. Both of thebeaded adsorbent products, (Samples G and H), were 1.0 mm in diameter.The breakthrough tests were carried out at a temperature of 90 degreesKelvin, and a pressure of 68 psig. The liquid argon feed originallycontained 100 parts per million of oxygen.

TABLE 3 Pilot Plant Performance Data* O₂ Dynamic Capacity Binder % wtBreakthrough at 1 part Adsorbent (Dry Weight Time to 1 part per permillion Type Binder Type Basis) million O₂ (min) (% wt) Sample H¹Mixture of 15-20 267 0.33 attapulgite, kaolin, bentonite Sample G²Actigel 12 1022 1.17 *The average particle diameter of all adsorbentswas 1.0 mm. ¹= Commercially available adsorbent ²= Laboratory preparedadsorbent

The importance of the binder type and content for the present process isconfirmed by the above performance data shown in Table 3. The comparisonclearly shows that the oxygen dynamic capacity of Sample G is 3.5×greater than that of Sample H and therefore provides improved processpurification performance. Since the binder content of Sample G isapproximately 6% less than that of Sample H, one would expect animprovement in the equilibrium adsorption capacity. However, the 3.5×improvement in the dynamic capacity shown for Sample G would not bepredicted simply by the difference in binder content between the twomaterials.

EXAMPLE 9 Preparation of Sample I (42% Lithium Exchange of Laboratory4A+12% Actigel®—Tilted Rotating Drum Mixing)

Sample I was prepared in a similar fashion to that of Example 7 (SampleG) and then, it was partially ion exchanged with lithium using thefollowing procedure.

On a dry weight basis, 12.53 lbs. of zeolite 4A powder (16.06 lbs. wetweight) was mixed with 1.71 lbs. of Actigel 208 (2.14 lbs. wet weight)in a Littleford LS-150 plow mixer for 10 minutes. The plow mixedpowdered intermediate powder mixture was transferred to a tiltedrotating drum mixer having internal working volume of ˜75 L and agitatedtherein at a speed of 24 rpm. Mixing of the formulation was continuedwhile adding de-ionized water gradually to form beads. A recyclingoperation was performed, involving grinding-up and reforming the beadsuntil the beads exhibited a porosity, which was measured using aMicromeritics Autopore IV Hg porosimeter on the calcined product, in therange of 30 to 35 percent. At the end of this mixing time period,beads—including those in the target 16×20 mesh size range, were formed.The product beads were then air dried overnight prior to calcinationusing the shallow tray method at temperatures up to 593 degreesCentigrade, as previously described in Example 3. The calcined beadswere subjected to a screening operation, both to determine yield andalso to harvest those particles that fell within the 16×20 mesh sizerange. The adsorbent particles were activated under dry air purge (flowrate 200 SCFH) using the same shallow tray calcination methodspreviously described.

Lithium ion exchange of the samples (to a Li ion exchange level of 42percent on a charge equivalent basis) was achieved using the followingprocedure; a batch ion exchange process was used where 450 g of thesample on a dry weight basis was placed inside a glass beaker andstirred in a 1.5 L lithium chloride solution (0.95 M) at 90 degreesCentigrade for 2 hours. This was followed by stirring the sample in 2Liters of de-ionized water at 90 degrees Centigrade for 15 minutes toremove excess lithium chloride. The exchange and wash process wasrepeated twice. Finally, the sample was packed in a glass column andwashed with de-ionized water, similar to the procedure described inExample 3, to fully remove any excess lithium chloride. The wet sampleswere dried, rescreened to 16×20 sized mesh, and activated under dry airpurge (flow rate 200 SCFH) again using the shallow tray calcinationmethod described in Example 3.

Effect of Lithium Ion Exchange of Zeolite 4A in Process Performance

Evidence from testing indicates that argon also adsorbs in themicropores of zeolite 4A, but not as easily, and at a much lowerobserved rate than that of oxygen. The first experimental indication wasobtained from single component McBain test data which showed acontinuous increase in the argon uptake at 87 degrees Kelvin over 480minutes during the adsorption test. Copending application entitled“Adsorbent Composition for Argon Purification” co-filed on Mar. 1, 2013as Dckt. No. 13235 and incorporated herein by reference in its entirety,further describes the composition of the adsorbent(s) used in thisprocess.

Breakthrough experiments under process relevant conditions have shownthat the oxygen capacity of the 4A zeolite decreased by pre-exposing thefreshly regenerated and indirectly cooled adsorbent to liquid argon. Onepurpose of these experiments was to simulate the conditions expected tooccur for an industrial process utilizing a much longer adsorbent bed(e.g. 20 feet or more) rather than the prototype bed used in the pilot.When a much longer bed is utilized for the purification process, oneskilled in the art understands that the portion of the bed close to thebed outlet is contacted with almost purified liquid argon for a longperiod of time (equal to the purification step time). Therefore, even ifargon enters the micropores of the adsorbent at a much slower rate thanoxygen, there is enough time at long cycle times, which are preferablefor the current invention, for argon to adsorb at portions of theadsorbent bed close to the outlet. This argon adsorption on theadsorbent bed will in turn sacrifice the bed performance for oxygenadsorption. Hence, an adsorbent with minimum argon uptake is preferable.The experiments presented on Table 4 were performed in the pilot plantdescribed above using a three foot long adsorbent bed. The processpressure and temperature during the purification stage of all tests were67 psig and 90 degrees Kelvin, respectively. The feed flow rate was 20slpm and the oxygen concentration in the argon stream was initially 100parts per million.

TABLE 4 Pilot Plant Performance Data* Percentage Lithium Pre-exposuretime O₂ Dynamic Capacity at Adsorbent Ion Exchange to liquid Ar (hr) 1part per million (wt %) Sample G² 0 1.0 1.07 Sample G² 0 48 0.35 SampleI² 42 1.0 2.5 Sample I² 42 48 2.0 *The average particle diameter of alladsorbents was 1.0 mm. ²= Laboratory prepared adsorbent

Table 4 shows that, under these process conditions, the laboratoryzeolite 4A sample (Sample G) lost 67 percent of its oxygen dynamiccapacity after 48 hours of exposure to liquid argon prior to the oxygenbreakthrough test. However, the 42 percent lithium exchanged laboratoryzeolite A (Sample I) lost only 20 percent of its dynamic oxygen capacityafter 48 hours of exposure to liquid argon prior to oxygen breakthroughtesting. Therefore, the oxygen capacity is decreased to a much lesserextent following pre-exposure of a freshly regenerated and indirectlycooled 42 percent lithium exchanged 4A adsorbent to liquid argon thanusing a 4A adsorbent. If the adsorbent is not lithium ion exchanged, theadsorbent bed must be increased in size or a more frequent regenerationwill be required to achieve the same argon purity results with the sameprocess constraints. In addition, it has been determined that when thezeolite 4A is ion exchanged with 42 percent lithium on a chargeequivalent basis (as for Sample I), the resulting material exhibits anincrease in oxygen capacity.

Various modifications and changes may be made with respect to theforegoing detailed description and certain embodiments of the inventionwill become apparent to those skilled in the art, without departing fromthe spirit of the present disclosure.

What is claimed is:
 1. An adsorption process for purifying a feed streamcontaining primarily liquid argon and oxygen employing an adsorbentexhibiting a faster rate of adsorption of oxygen and a slower rate ofargon under the liquid argon feed conditions, comprising the followingcycle of process steps: a) supplying from the inlet of an adsorbent bedsaid liquid argon feed that contains oxygen in an amount of more than 10parts per million and less or equal to 10,000 parts per million,adsorbing at least part of the oxygen on the adsorbent, at or belowcryogenic temperatures, thereby producing a purified liquid argonproduct leaving said adsorbent bed from the outlet with less than orequal to 10 parts per million oxygen present in said purified liquidargon product; b) draining from said adsorbent bed purified residualliquid argon by introducing a displacement purge gas; c) allowing saidadsorbent bed containing said adsorbent to warm to a temperature,desorbing at least part of the adsorbed oxygen and removing saidadsorbed oxygen from the adsorbent bed such that the liquid argon feedmay be supplied for purposes of repeating the cycle; d) indirectlycooling said adsorbent bed having an inlet and an outlet and containingan adsorbent such that said adsorbent bed is indirectly cooled to atemperature below the boiling point of argon; e) wherein said processsteps (a)-(d) are repeated in a cyclical manner.
 2. The process of claim1, wherein the liquid argon feed for step (a) contains oxygen in theconcentration range of about 10 parts per million and wherein removal ofsaid oxygen from said liquid argon feed results in a purified liquidargon product with less than or equal to 1 parts per million of oxygen.3. The process of claim 1, wherein the liquid argon feed temperature forstep (a) is less than the boiling point of argon and the feed pressureis greater than or equal to 20 psig.
 4. The process of claim 1, whereinthe purified residual liquid argon is drained from the adsorbent bed instep (b) using a nitrogen purge.
 5. The process of claim 1, wherein thepurified residual liquid argon is drained from the adsorbent bed in step(b) using an argon purge.
 6. The process of claim 1, wherein the warmingof the adsorbent bed in step (c) is carried out by first removing theliquid nitrogen used for indirect cooling and subsequently by purgingthe adsorbent bed with gaseous nitrogen having a temperature of at least200 degrees Kelvin and a pressure of at least 2 psig.
 7. The process ofclaim 6, wherein the warming of the adsorbent bed in step (c) iscontinued until the adsorbent bed reaches a temperature of at least 200degrees Kelvin.
 8. The process of claim 7, wherein following the warmingof the adsorbent bed, a gaseous argon purge having a temperature of atleast 200 degrees Kelvin and a pressure of at least 2 psig is carriedout until the effluent from the bed is predominantly argon.
 9. Theprocess of claim 1, wherein the warming of the adsorbent bed in step (c)is carried out by first removing the liquid nitrogen used for indirectcooling and subsequently by purging the adsorbent bed with gaseous argonhaving a temperature of at least 200 degrees Kelvin and a pressure of atleast 2 psig.
 10. The process of claim 9, wherein the warming of theadsorbent bed in step (c) is continued until the adsorbent bed reaches atemperature of at least 200 degrees Kelvin.
 11. The process of claim 1,wherein cooling step (d) is carried out firstly by indirect coolingusing liquid nitrogen until the adsorbent bed reaches a temperature ofless than about 150 degrees Kelvin and subsequently by direct coolingusing liquid argon wherein the cooling step is complete when theadsorbent bed, containing adsorbent, sustains the argon feed in a liquidphase.
 12. The process of claim 11, wherein the liquid argon for use incooling step (d) is at least partially recovered from said adsorbent bedduring the purification step (a).
 13. The process of claim 1, whereinthe dynamic capacity of the adsorbent for oxygen to 1 part per millionof oxygen of step (a) of subsequent adsorption cycles is at least 80percent of said dynamic capacity of the adsorbent for oxygen for step(a) of the first adsorption process cycle.
 14. The process of claim 1,further comprising a second adsorbent bed wherein said second adsorbentbed is operated such that it is purifying liquid argon feed in step (a)while the first adsorbent bed is being regenerated by steps (b), (c) andcooled by step (d) and correspondingly the second adsorbed bed isregenerated by steps (b), (c) and cooled by step (d) while said firstadsorbent bed is purifying the liquid argon feed in step (a), so as toproduce a purified liquid argon product stream continuously.